A Digital Television Standard published Sep. 16, 1995 by the Advanced Television Systems Committee (ATSC) specifies vestigial sideband (VSB) signals for transmitting digital television (DTV) signals in 6-MHz-bandwidth television channels. DTV signals will be transmitted in certain of the ultra-high-frequency transmission channels currently used in over-the-air broadcasting of National Television System Committee (NTSC) analog television signals within the United States. The VSB DTV signal is designed so its spectrum is likely to interleave with the spectrum of a co-channel interfering NTSC analog TV signal. The symbol frequency of the DTV signal is three times NTSC color subcarrier frequency, which 3.58 MHz subcarrier frequency is 455/2 times NTSC scan line rate. The pilot carrier and the principal amplitude-modulation sideband frequencies of the DTV signal are positioned at odd multiples of one-quarter the horizontal scan line rate of the NTSC analog TV signal. This causes these DTV signal components to fall between the even multiples of one-quarter the horizontal scan line rate of the NTSC analog TV signal, at which even multiples most of the energy of the luminance and chrominance components of a co-channel interfering NTSC analog TV signal will fall. The video carrier of an NTSC analog TV signal is offset 1.25 MHz from the lower limit frequency of the television channel. The carrier of the DTV signal can be offset from such video carrier by 59.75 times the horizontal scan line rate of the NTSC analog TV signal, to place the carrier of the DTV signal about 309,877.6 kHz from the lower limit frequency of the television channel. Accordingly, the carrier of the DTV signal is about 2,690122.4 Hz from the middle frequency of the television channel.
The exact symbol rate in the Digital Television Standard is (684/286) times the 4.5 MHz sound carrier offset from video carrier in an NTSC analog TV signal. The number of symbols per horizontal scan line in an NTSC analog TV signal is 684, and 286 is the factor by which NTSC horizontal scan line rate is multiplied to obtain the 4.5 MHz sound carrier offset from video carrier in an NTSC analog TV signal. The symbol rate is 10.762238 million symbols per second, which can be contained in a VSB signal extending 5.381119 MHz from DTV signal carrier. That is, the VSB signal can be limited to a band extending 5.690997 MHz from the lower limit frequency of the television channel.
The ATSC standard for DTV signal terrestrial broadcasting in the United States of America is capable of transmitting either of two high-definition television (HDTV) formats with 16:9 aspect ratio. One HDTV display format uses 1920 samples per scan line and 1080 active horizontal scan lines per 30 Hz frame with 2:1 field interlace. The other HDTV display format uses 1280 luminance samples per scan line and 720 progressively scanned scan lines of television image per 60 Hz frame. The ATSC standard also accommodates the transmission of DTV display formats other than HDTV display formats, such as the parallel transmission of four television signals having normal definition in comparison to an NTSC analog television signal.
DTV transmitted by vestigial-sideband (VSB) amplitude modulation (AM) during terrestrial broadcasting in the United States of America comprises a succession of consecutive-in-time data fields each containing 313 consecutive-in-time data segments. One may consider the data fields to be consecutively numbered modulo-2, with each odd-numbered data field and the succeeding even-numbered data field forming a data frame. The frame rate is 20.66 frames per second. Each data segment is of 77.3 microseconds duration. So, with the symbol rate being 10.76 MHz, there are 832 symbols per data segment. Each segment of data begins with a data-segment-synchronization (DSS) code group of four symbols having successive values of +S, -S, -S and +S. The value +S is one level below the maximum positive data excursion, and the value -S is one level above the maximum negative data excursion. The initial data segment of each data field includes a data-field-synchronization (DFS) code group that codes a training signal for channel-equalization and multipath suppression procedures. The training signal is a 511-sample pseudo-noise sequence (or "PN-sequence") followed by three 63-sample PN sequences. The middle ones of the 63-sample PN sequences in the DFS codes are transmitted in accordance with a first logic convention in the first line of each odd-numbered data field and in accordance with a second logic convention in the first line of each even-numbered data field. The first and second logic conventions are complementary to each other (i. e., of opposite senses of polarity).
The data within data segments are trellis coded using twelve interleaved trellis codes, each a 2/3 rate trellis code with one uncoded bit that is precoded. The interleaved trellis codes are subjected to Reed-Solomon forward error-correction coding, which provides for correction of burst errors arising from noise sources such as a nearby unshielded automobile ignition system. The Reed-Solomon coding results are transmitted as 8-level (3 bits/symbol) one-dimensional-constellation symbol coding for over-the-air transmission. The Reed-Solomon coding results are transmitted as 16-level (4 bits/symbol) one-dimensional-constellation symbol coding for cablecast, which transmissions arc made without any preceding after symbol generation. The VSB signals have their natural carrier wave, which would vary in amplitude depending on the percentage of modulation, suppressed.
The natural carrier wave is replaced by a pilot carrier wave of fixed amplitude, which amplitude corresponds to a prescribed percentage of modulation. This pilot carrier wave of fixed amplitude is generated by introducing a direct component shift into the modulating voltage applied to the balanced modulator generating the amplitude-modulation sidebands that are supplied to the filter supplying the VSB signal as its response. If the eight levels of 4-bit symbol coding have normalized values of -7, -5, -3, -1, +1, +3, +5 and +7 in the carrier modulating signal, the pilot carrier has a normalized value of 1.25. The normalized value of +S is +5, and the normalized value of-S is -5.
In the earlier development of the DVT art it was contemplated that the DTV broadcaster might be called upon to decide whether or not to use a symbol precoder at the transmitter. This symbol precoder would follow the symbol generation circuitry and, when used together with a comb filter in each DTV signal receiver, would provide for matched filtering of symbols. The comb filter would precede the data-slicer in the symbol decoder circuitry of the DTV signal receiver and would operate as a symbol post-coder. This decision would have depended upon whether interference from a co-channel NTSC broadcasting station were expected or not. Symbol precoding would not have been used for data-segment-synchronization code groups or during data segments in which data field synchronization data were transmitted. Co-channel interference is reduced at greater distances from the NTSC broadcasting station(s) and is more likely to occur when certain ionospheric conditions obtain, the summertime months during years of high solar activity being notorious for likelihood of co-channel interference. Such interference will not obtain if there are no co-channel NTSC broadcasting stations, of course. If there were likelihood of NTSC interference within his area of broadcast coverage, it was presumed that the HDTV broadcaster would use the symbol precoder to facilitate the HDTV signal being more easily separated from NTSC interference. Accordingly, a comb filter would be employed as symbol post-coder in the DTV signal receiver to complete matched filtering. It was presumed that the DTV broadcaster would discontinue using the symbol precoder if there were little or no possibility of NTSC interference. Accordingly, the symbol post-coder in each DTV signal receiver would then be disabled, in order that flat spectrum noise would be less likely to cause erroneous decisions as to symbol values in the trellis decoder.
U.S. Pat. No. 5,260,793 issued Nov. 9, 1993 to R. W. Citta et alii and entitled "RECEIVER POST CODER SELECTION CIRCUIT" selectively employs a post-coder comb filter. The filter suppresses artifacts of co-channel NTSC interference accompanying a real or in-phase baseband component (I channel) of the complex output signal of a demodulator used in a DTV signal receiver. The presence of these artifacts in the I-channel component of the demodulator response is detected for developing control signals automatically to enable or disable the suppression of the artifacts of co-channel NTSC interference by comb filtering. During each data field sync interval, the input signal to and the output signal from an NTSC suppression filter of comb filter type in the DTV signal receiver are each compared with a respective signal that is known a priori and is drawn from memory within the HDTV signal receiver. If the minimum result of comparison with the input signal has less energy than the minimum result of comparison with the output signal from the NTSC suppression filter, this is indicative that the primary cause of variance from expected reception is random noise rather than co-channel NTSC interference. Insofar as the particular DTV signal receiver is concerned, reception would be better were precoding and post-coding not employed in the system, and it is presumed that the broadcaster has not employed preceding. If the minimum result of comparison with the input signal has more energy than the minimum result of comparison with the output signal from the NTSC suppression filter, this is indicative that artifacts of co-channel NTSC interference rather than random noise are the primary cause of variance from expected reception. Insofar as the particular DTV signal receiver is concerned, reception would be better were precoding and post-coding employed in the system, and it is presumed that the broadcaster has employed preceding.
U.S. Pat. No. 5,546,132 issued Aug. 13, 1996 to K. S. Kim et alii and entitled "NTSC INTERFERENCE DETECTOR" describes the use of post-coder comb filtering for suppressing artifacts of co-channel NTSC interference when the presence of such interference is detected in NTSC-extraction comb filter response to the I channel. U.S. Pat. No. 5,546,132 does not specifically describe an imaginary or quadrature-phase baseband component (Q channel) of a complex output signal being supplied from the demodulator used in a DTV signal receiver. A DTV signal receiver that synchrodynes the VSB AM signals to baseband commonly employs a demodulator that includes an in-phase synchronous detector for supplying received I-channel signal for trellis decoding (after post-coding, if precoding is used at the transmitter). This demodulator further includes a quadrature-phase synchronous detector for supplying received Q-channel signal which is lowpass filtered to generate an automatic frequency and phase control (AFPC) signal for the local oscillator supplying carrier for synchrodyning. The specification and drawing of U.S. Pat. No. 5,479,449 issued Dec. 26, 1996 to C. B. Patel and A. L. R. Limberg, entitled "DIGITAL VSB DETECTOR WITH BANDPASS PHASE TRACKER, AS FOR INCLUSION IN AN HDTV RECEIVER", and assigned to Samsung Electronics Co., Ltd., is incorporated herein by reference. The reader's attention is specifically directed to elements 22-27 in FIG. 1 of the drawing of U.S. Pat. No. 5,479,449 and the description thereof in the accompanying specification. These elements are used in the described DTV signal receiver for carrying out complex demodulation of the VSB AM final intermediate-frequency signal. U.S. Pat. No. 5,479,449 describes complex demodulation of the VSB AM final I-F signal being carried out in the digital regime, but in alternative DTV receiver designs complex demodulation of the VSB AM final I-F signal is instead carried out in the analog regime.
In both U.S. Pat. Nos. 5,260,793 and 5,546,132 post-coding is enabled during times of substantial co-channel NTSC interference and otherwise disabled, with the control signal for such selective enablement being developed from the received I-channel signal. The determination of co-channel NTSC interference levels is complicated by the direct bias accompanying the co-channel NTSC interference. The direct bias arises from the in-phase synchronous detection of the pilot carrier of the VSB AM DTV signal. This is particularly a problem in DTV signal receivers in which automatic gain control does not tightly regulate the amplitude of the received I-channel signal recovered by in-phase synchronous detection.
The video carrier of an NTSC signal is 1.25 MHz from edge of the 6-MHz-wide broadcast channel, while the carrier for a DTV signal for terrestrial through-the-air broadcast is 310 kHz from edge of the 6-MHz-wide broadcast channel. A co-channel NTSC signal does not exhibit symmetrical amplitude-modulation sidebands with respect to the carrier of the vestigial-sideband amplitude modulation (VSB AM) carrying digital information. Accordingly, artifacts of the NTSC video carrier at 940 kHz remove from DTV signal carrier and artifacts of its sidebands arc not well canceled in the DTV signal as synchrodyned to baseband. Nor, of course, are artifacts of the NTSC audio carrier and its sidebands, the NTSC audio carrier being at 5.44 MHz remove from DTV signal carrier.
The Digital Television Standard the ATSC published Sep. 16, 1995 does not allow for the use of precoding of complete data symbols at the DTV transmitter to compensate for post-coding incidental to subsequent use of comb filtering in a DTV signal receiver to reject co-channel NTSC interference. Instead, only the initial bit in each symbol is precoded. This procedure by itself does not facilitate a DTV signal receiver using comb filtering to reject co-channel NTSC interference before data slicing procedures are undertaken. A DTV signal receiver that does not reject artifacts of co-channel NTSC interference before data slicing procedures are undertaken will not have good reception under strong co-channel NTSC interference conditions. Such conditions may be caused by the DTV signal receiver being remote from the DTV transmitter or having a close-by analog TV transmitter. In the DTV signal as synchrodyned to baseband the artifacts of the video carrier of a co-channel interfering NTSC color TV signal are at 59.75f.sub.H, f.sub.H being the horizontal scan frequency of that signal. The artifact of the color subcarrier is at 287.25f.sub.H, and the artifact of the unmodulated NTSC audio carrier is at 345.75f.sub.H.
Comb filtering procedures are not entirely satisfactory for suppressing artifacts of the frequency-modulated NTSC audio carrier, particularly under conditions of frequency modulation in which carrier frequency deviation is large, the inventor points out. This is because correlation (or anti-correlation) of samples of the FM carrier at times separated by any substantial fixed delay may not be particularly good. U.S. Pat. No. 5,748,226 entitled "DTV RECEIVER WITH FILTER IN I-F CIRCUITRY TO SUPPRESS FM SOUND CARRIER OF CO-CHANNEL NTSC INTERFERING SIGNAL" issued to the inventor on May 5, 1998 is incorporated herein by reference. In U.S. Pat. No. 5,748,226 the inventor recommends that the filtering used to establish the overall bandwidth of intermediate-frequency amplification be such as to reject the FM audio carrier of any co-channel interfering NTSC analog TV signal.
Comb filtering procedures are more satisfactory for separating the baseband DTV signal from other artifacts of the co-channel NTSC signal arising from the video carrier, the low video frequencies, and the chrominance signal frequencies close to the color camer. This is because these artifacts tend to exhibit good correlation between samples separated by certain specific delay intervals and good anti-correlation between samples separated by certain other specific delay intervals.
In U.S. Pat. No. 5,748,226 the inventor advocates preceding data-slicing in a DTV signal receiver with comb filtering to suppress co-channel NTSC interference when it is large enough to affect data-slicing adversely. The inventor teaches how to compensate in the symbol decoding procedure for the effects of such comb filtering upon symbol coding when it is selectively done. It is still useful, then, to be able to determine when co-channel NTSC interference is larger than a prescribed value denominated as being acceptably small, so that this determination can be used for controlling the selective use of comb filtering to suppress co-channel NTSC interference.
Co-channel NTSC interference will appear in the imaginary or quadrature-phase baseband component (Q channel) of the complex output signal of a demodulator used in a DTV signal receiver whenever co-channel NTSC interference appears in the real or in-phase baseband component (I channel) of that complex output signal. Accordingly, an NTSC interference detector can be arranged so that its NTSC extracting filter responds to the received Q-channel signal, rather than the received I-channel signal. The amount of co-channel NTSC interference is significant if it causes too many errors in the trellis decoding of equalized received I-channel signal to be corrected by the Reed-Solomon decoder following the trellis decoder. By determining whether or not a significant amount of co-channel NTSC interference accompanies the received Q-channel signal, it is inferentially determined whether or not a significant amount of co-channel NTSC interference accompanies the received I-channel signal. The accurate determination of co-channel NTSC interference levels tends to be simpler, because essentially no direct bias arises from the quadrature-phase synchronous detection of the pilot carrier of the VSB AM DTV signal after the synchronous detection apparatus has achieved phase-lock with the pilot carrier.
A co-channel NTSC interference detector that is insensitive to direct bias arising from synchronous detection of the pilot carrier was an objective of the inventor when developing apparatus disclosed in this specification. Such a co-channel NTSC interference detector permits direct determination of whether or not a significant amount of co-channel NTSC interference accompanies the received I-channel signal without need for an equalization filter that suppresses direct bias arising from synchronous detection of the pilot carrier. Such an equalization filter is more difficult to implement than an equalization filter that has response at zero frequency. Also, an equalization filter without response at zero-frequency can interfere with automatic-gain-control (AGC) and automatic-frequency-and-phase-control (AFPC) loops in certain DTV signal receiver designs. In a DTV signal receiver that responds to the received Q-channel signal to determine whether or not co-channel NTSC interference is significant, a co-channel NTSC interference detector that is insensitive to direct bias arising from synchronous detection of the pilot carrier is still useful. Such co-channel NTSC interference detector provides for continuity in the initial adjustment of DTV signal receiver equalization.